1. Field of the Invention
The present invention relates to a III-V compound semiconductor field-effect transistor.
2. Related Background Art
In semiconductor device art, field-effect transistors made of III-V compound semiconductor materials such as GaAs and InP are known. Since GaAs and InP have smaller electron effective masses than that of silicon (Si), they have high electron mobility. These materials are suitable for high-frequency devices.
Available high-frequency devices include voltage-controlled elements such as a field-effect transistor (FET) having an active layer of GaAs semiconductor and a high-electron-mobility transistor (HEMT), and current-controlled elements such as a hetero-bipolar transistor (HBT).
The inventors have found the following problem in the course of studying high-frequency devices.
FIG. 1 shows the dependence of the electron drift velocities on electric field strength in GaAs, InP, and Si semiconductors. The slopes of characteristic curves shown in FIG. 1 indicate the electron mobility of these materials. GaAs and InP have mobility three to five times as large as that of Si.
GaAs and InP, however, indicate the characteristics as follows. As the field strength increases, their electron drift velocities rise to the respective maximum values and decrease. When the electric field strength further increases, the carriers lose the kinetic energy by optical phonon scattering. Due to this scattering, the electron drift velocity approaches a certain value, e.g., approximately 1.0xc3x97107 cm/s, which is independent of the electric field strength. The advantage of the higher mobility in such materials is lost in the following two cases: (i) the GaAs and InP devices operate under a high applied voltage; and (ii) the device dimensions are reduced, so that the internal electric field increases.
In GaAs, the avalanche breakdown phenomenon occurs at relatively low voltage. The avalanche breakdown voltage relates to the internal electric field in semiconductor, and the voltage is inversely proportional to the impurity concentration. If the impurity concentration of a GaAs active layer is about 1.0xc3x971018 cmxe2x88x923, only an avalanche breakdown voltage of about 30 V can be achieved in commercially available devices. The inventors have found the following problem: when device dimensions decrease, not only the electron velocity but also the breakdown voltage of the device lowers under device operation.
On the other hand, InP is a material having smaller electron mobility than that of GaAs, but its maximum electron drift velocity is higher than that of GaAs semiconductor. InP semiconductor FETs also has been developed. InP semiconductor can not attain Schottky barrier height enough to be applied to MESFET, and also can not be applied to MIS FET or MOS FET because no semiconductor material and insulating film suitable for these FET are not found.
To avoid this problem, an attempt has been made to apply semiconductor having a composition of Ga0.51In0.49P to an active layer. In this semiconductor, a number of In cites in an InP semiconductor crystal are replaced with Ga atoms. This Ga0.51In0.49P has the following characteristics: (a) an energy gap value of 1.9 eV; (b) an electron effective mass larger than that of GaAs semiconductor. Avalanche breakdown voltages depend upon the energy gap values of semiconductors. Under the same impurity concentration, the larger the energy gap is, the higher the avalanche breakdown voltage becomes. A Ga0.51In0.49P semiconductor FET has an avalanche breakdown voltage of 50 V or more. With this semiconductor, the device can achieve a high avalanche breakdown voltage. Ga0.51In0.49P semiconductor, however, has smaller mobility than that of GaAs semiconductor because of its effective mass. The inventors have found that, under high electric field, GaInP semiconductor attains electron drift velocity nearly equal to that of GaAs. Therefore, in the high-frequency performance, the Ga0.51In0.49P device is about as high as a GaAs semiconductor device.
It is an object of the present invention to provide a device having a high avalanche breakdown voltage and high performance in high-frequency regions.
A field-effect transistor according to the present invention includes a channel layer and a gate electrode. The channel layer has a first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer (0 less than x less than 1, 0xe2x89xa6y less than 1) and a GazIn1xe2x88x92zAs layer (0 less than zxe2x89xa61). The gate electrode is provided so as to control a channel current flowing in the channel layer.
Since the channel layer includes the first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer and the GazIn1xe2x88x92zAs layer, this GazIn1xe2x88x92zAs layer serves as a main channel when the applied voltage is low. As the applied voltage is increased, carriers begin moving to the xcex93-valley energy level of the first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer, not to the L-valley energy level of the GaInAs layer. When the applied voltage is high, the first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer serves as a main channel.
Additionally, the first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer has a larger energy gap than that of the GazIn1xe2x88x92zAs layer. When the applied voltage is high, the carriers conduct in the first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer having a relatively large avalanche breakdown voltage.
In the present invention, any features in accordance with the present invention as described below can be arbitrarily combined with each other.
The field-effect transistor according to the present invention can further include a first AluGa1xe2x88x92uAs layer (0xe2x89xa6u less than 1). The first AluGa1xe2x88x92uAs layer (0xe2x89xa6u less than 1) can be provided between a substrate and the channel layer.
The field-effect transistor according to the present invention can further include a second AlvGa1xe2x88x92vAs layer (0xe2x89xa6v less than 1). The second AlvGa1xe2x88x92vAs layer (0xe2x89xa6v less than 1) can be provided between the gate electrode and the channel layer. This can increase the barrier height against the gate electrode.
In the field-effect transistor according to the present invention, the first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer can contain donor impurities. The GazIn1xe2x88x92zAs layer has smaller donor impurity concentration than that of said first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer.
In the field-effect transistor according to the present invention, the first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer can be in contact with the GazIn1xe2x88x92zAs layer (0 less than zxe2x89xa61). The first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer can also have a lattice constant different from that of GaAs semiconductor. This is the case where the GazIn1xe2x88x92zAs layer preferably has a thickness equal to or smaller than the critical thickness.
In the field-effect transistor according to the present invention, the first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer can have substantially the same lattice constant as a GaAs layer.
In the field-effect transistor according to the present invention, the channel layer can further have a second GapIn1xe2x88x92pAsqP1xe2x88x92q layer (0 less than p less than 1, 0xe2x89xa6q less than 1). The GazIn1xe2x88x92zAs layer (0 less than zxe2x89xa61) can be provided between the first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer and the second GapIn1xe2x88x92pAsqP1xe2x88x92q layer.
In the field-effect transistor according to the present invention, the second GapIn1xe2x88x92pAsqP1xe2x88x92q layer can have substantially the same lattice constant as GaAs semiconductor.
In the field-effect transistor according to the present invention, the channel layer can include a GatIn1xe2x88x92tP layer (0 less than t less than 1) and a GaAs layer.
In the field-effect transistor according to the present invention, the substrate can be a GaAs substrate. The first GaxIn1xe2x88x92xAsyP1xe2x88x92y layer can also have substantially the same lattice constant as GaAs semiconductor.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.